In 19th/20th century London there used to be Cats' Meat Men who went door to door selling meat unfit for human consumption to feed your pets- often it was horsemeat .http://www.museumoflondonprints.com/image/68598/john-galt-cats-meat-man-c-1901

Butchers would (probably still will) also sell as pet food cheap gristly mince or parts of an animal not usually eaten by people. My mother for example used to buy "lights" - cows' lungs -and boil them up for the cat. This was the 1960's/70's, tinned food was definitely around, but I suppose lights were cheaper.